An infrared (IR) image detector forms an image by detecting radiation in the infrared portion of the electromagnetic spectrum from the imaged scene. A passive infrared detector operates by measuring the infrared radiation emitted by the sources, particularly thermal energy in the far infrared range, as opposed to active IR detectors which first illuminates the objects with IR radiation and then captures the reflections of the illuminations. The inherent spatial resolution of a detector with a two-dimensional matrix array is a function of the size and number of pixels within the array (the pixel density). For many types of image sensors, such as complementary metal-oxide-semiconductor (CMOS)-based or charge-coupled device (CCD)-based sensors, it is fairly straightforward to increase the resolution by adding more pixels within a fixed space and/or decreasing pixel size in the array. However, for IR sensors such an approach would be prohibitively difficult and expensive. Furthermore, IR detectors are prone to receiving scarce light (particularly with passive IR detectors), and usually requires manipulations or treatment in order to enhance the amount of radiation collected by the detector to enable effective imaging in the IR wavelength range.
U.K. Patent No. 2,270,230 to Hirose, entitled “Two dimensional image detector”, discloses masking the pixels of a sensor array such that a subdivided region is imaged by each pixel. A mask is provided in opposition to a surface of the sensor array, where the mask includes windows having a smaller area than the array pixels. A mask shifter changes the positions of the windows at pitches smaller than those between the pixels. Light passing through the mask window and into each pixel in the sensor array is divided into a desired number of subdivisions as the mask changes position, thus increasing the spatial resolution of the image relative to the pixel size. The mask may be implemented via a metal mask or via electronic manipulation of a substance having polarizing characteristics, such as liquid crystals.
European Patent No. 1,198,119 to Parsons, entitled “Improved resolution for an electric image sensor array”, discloses introducing a partially occluding mask/light source between the sensor array and the image. Resolution improvement is obtained by displacement between the array and the mask/light source and calculating grey scale values for the improved resolution from the different changes in the charges of the individual array sensors.
PCT Application Publication No. 98/46007 to Bone, entitled “Imaging system and method”, is directed to improving the resolution of a charge coupled device having a two-dimensional array of light sensitive detector elements. Discrete areas of each detector element are sequentially masked with an apertured opaque mask chosen so as to allow for square (n×n) arrays of aperture elements for each detector and so that the apertures can be arranged as an embedded set which facilitate hierarchical refinement of the resolution.
U.S. Pat. No. 6,005,682 to Wu et al, entitled “Resolution enhancement by multiple scanning with a low-resolution, two-dimensional sensor array”, is directed to high-resolution imaging with low-resolution two-dimensional imagers whose sensors are only a fraction of a selected pixel area. Sensors are stepped across an image with an optical or mechanical stepper to acquire an image at each sensor position. Multiple images are obtained from individual sensors, which have a sensed area substantially less than the area of the pixels of the sensor array. The rescanning is accomplished by movable mirrors or lenses that are disposed between an illuminated specimen and a sensor area.
U.S. Pat. No. 5,712,685 to Dumas, entitled “Device to enhance imaging resolution”, is directed to improving resolution of a Focal Plane Array (FPA) image sensor with microscanning methodology. A mask having a checkerboard pattern of opaque and transparent areas is located in front of and adjacent to the FPA onto which an image of a scene is optically focused. The mask and FPA are displaced with respect to each other in a number of micro-steps that extend in the same direction as the columns/rows of the FPA grid. The micro-steps are of equal lengths and are each a fraction of the length of a detector element. Opaque areas of the mask progressively cover equal areas of detector elements in discrete steps for each micro-step movement in one direction at the same time as transparent areas of the mask progressively uncover equal areas of other detector elements for each discrete step. Outputs from the detector elements provide signals for sample slices of the scene for each micro-step, and an image of the scene is reconstructed from the signals.